1. Field of the Invention
This invention concerns means and method for reducing waste of power in centrifuges of the type having a rotating bowl in which material flowable as a liquid forms an annular pool and is treated with high centrifugal forces and from which pool such material is discharged while the bowl rotates. More particularly, the invention provides such means and method which can be used to reduce such power waste in either or both reducing the power required to accelerate the material being fed to a treating zone in the outer portion of the bowl to the surface velocity of the pool, and to recover power so applied from the kinetic energy imparted thereby to material discharging from the bowl. While not limited thereto, the invention is particularly described as applied to centrifuges which continuously discharge separately fractions of the material segregating in the bowl, utilizing a conveyor in the discharge of at least one of the fractions.
2. Description of the Prior Art
In feeding material into the outer treating zone of a centrifuge bowl rotating about an axis at high speed, the material must in some way be accelerated to the velocity of the pool surface in the treating zone. The power necessary to effect such acceleration is herein referred to as "hydraulic power", as distinguished from the mechanical power required to rotate the bowl empty, supplying torque necessary to overcome windage and friction losses and to drive the conveyor. If the feed is from a stationary pipe directly into the treating zone, the power for such feed acceleration is imparted thereto directly by rotation of the bowl, and is accompanied by high turbulence and high hydraulic power requirements. Various feed systems have been devised in the prior art to avoid such turbulence, but these have not resulted in major reductions of hydraulic power, which may in many cases constitute 50 percent or more of the total power requirements of the centrifuge.
One such system uses axial feed into a coaxial cone in the bowl with its larger end in or adjacent the treating zone, the feed attaining most of its velocity by frictional slip on the cone surface as it flows outwardly therein to the treating zone. Centrifuges of the rotary helical conveyor type have more usually fed the material from a stationary pipe into the conveyor hub, of smaller diameter than the treating zone of the bowl and rotating in the same direction at a small differential above or below the rate of rotation of the bowl. The partially accelerated feed is then fed to the treating zone from ports in the conveyor hub or, to reduce turbulence, by tubes or vanes providing a flow path to the zone and further accelerating the feed.
In such prior art systems all of the requisite acceleration of the feed is effected either by the bowl or by additional mechanism rotated about the bowl axis at, or substantially at, bowl speed and at low efficiency.
Various suggestions in the prior art for recovering power from the kinetic energy of material discharging from the bowl have not been suitable for, or considered sufficiently effective for, general adoption, and the practical art has continued for the most part to waste such kinetic energy by discharging the material directly to stationary receivers. Such prior art suggestions include the following.
U.S. Pat. No. 1,032,285 of 1912 and French Patent No. 876,531 of 1942 disclose centrifuges in which a liquid fraction is discharged through curved passages extending generally away from the bowl axis. Although the arrangement is intended to recover power by converting kinetic energy in the discharging material into usable shaft power applied to the bowl, net power recovery, if any, would be small, since extra power would have to be applied to the bowl to cause flow outwardly of the axis through the passages. Similarly, U.S. Pat. No. 3,791,577 of 1974 discloses a centrifuge in which a solid slurry fraction is discharged from the bowl end over a wide lip extending away from the bowl axis, from which lip it is ejected away from the bowl axis against curved baffle plates on a separately driven rotor having a different axis. Here, again, bowl rotation power required to be added to force the material away from the bowl axis would drastically reduce any net power savings from the arrangement, the stated purpose of which is to reduce product degradation, not to recover power.
U.S. Pat. No. 3,862,714 of 1975 discloses a centrifuge in a conical end provided with straight vanes, rotating with the bowl, which form with the bowl end straight passages that slant inwardly toward the bowl axis, through which the liquid fraction passes to an axial outlet near the apex of the conical part. The patent states that the liquid gives up acquired angular momentum to the vanes and to that extend reduces power requirements for rotating the bowl. The arrangement appears limited in use to the particular type of centrifuge disclosed in the patent, which is operated with the bowl full and with a forced vortex.
Yet another system proposed in the prior art relies on jetting liquid out holes in the bowl wall, as exemplified by U.S. Pat. No. 2,410,313 of 1946. Apart from adding undesirable complexity to bowl construction for what could at best be power recovery from only a small portion of the liquid, and difficulties of plugging of the necessarily small outlets with solids, the system is handicapped by inability to discharge the jets in the most effective tangential direction, for practical reasons set forth in said U.S. Pat. No. 2,410,313.